Determining the viscosity of a hydrocarbon reservoir fluid

ABSTRACT

Deteriming the viscosity of a hydrocarbon reservoir fluid that is present in a formation layer traversed by a borehole, which method comprises the steps of selecting a location in the formation layer; lowering in the borehole to the location a tool that comprises a central conduit having an inlet, means for displacing fluids through the central conduit, and an optical fluid analyser; making an exclusive fluid commuication between the formation and the inlet of the central conduit; obtaining a spectrum of the optical density; calculating a first factor that is the maximum optical density in a predetermined short-wavelenght range multiplied with the length of the short-wavelenght range, calculating a second factor which is the integral over the same short-wavelength range of the spectrum, subtracting the second factor from the first factor to obtain a hydrocarbon oil porperty; and obtaining the magnitude of the in situ viscosity from the oil property using a relation that had been obtained by fitting a curve ( 1 ) through previously obtained data points ( 2, 3, 4, 5 ) comprising the measured magnitude of the actual viscosity as a function of the oil property.

[0001] The present invention relates to determining the viscosity of ahydrocarbon reservoir fluid.

[0002] In order to measure the viscosity of a hydrocarbon reservoirfluid, a sample of the reservoir fluid is taken and analysed underreservoir pressure and temperature. A brief description of the way inwhich a PVT analysis is carried out is given in section 3 of the bookContributions in Petroleum Geology and Engineering, Volume 5, Propertiesof Oils and Natural Gases, K. S. Pederson et al, 1989. Such an analysiscan be very accurate, however it takes a long time to be completed.

[0003] It is of great importance to know the viscosity of the reservoirfluid as soon as possible, preferably directly after a well has beendrilled. Because, then there is still a possibility to adjust the designof the production and surface equipment to take into account the actualviscosity.

[0004] There are analysis tools, such as the modular dynamics formationtests from Schlumberger, the repeat dynamic tester from Halliburton andthe reservoir characterization instrument from Western Atlas that areprovided with an optical fluid analyser. Such an analyser operates bysubjecting the fluid to be analysed to an absorption spectroscopy in thevisible and near infrared ranges. The analyser measures thetransmittance (which is the ratio of transmitted light energy toincident light energy) at different wavelengths. The output of theanalyser is the optical density spectrum (which is the optical density,log(1/transmittance), as a function of wavelength).

[0005] Reference is made to SPE Paper 39093, Determination of produciblehydrocarbon type and oil quality in wells drilled with syntheticoil-based muds, M. N. Hashem et al, 1997. In this paper it is disclosedthat there is a correlation between the output of the analyser and theAPI gravity and between the output of the analyser and the gas-oilratio.

[0006] Reference is further made to SPE paper 63252, Determination ofhydrocarbon properties by optical analysis during wireline fluidsampling, A. van Dusen et al, 2000. This paper discloses that there is acorrelation between the output of the analyser and some of the PVTproperties, where PVT is an acronym used to refer to pressure, volumeand temperature. According to this publication, density, saturationpressure, oil compressibility, formation volume factor and gas-oil ratiogave a good correlation, and that weaker correlations were found withother PVT properties.

[0007] Applicant has surprisingly found that there is a good correlationbetween the viscosity and a particular combination of the analyseroutput.

[0008] Thereto the method of determining the viscosity of a hydrocarbonreservoir fluid that is present in a formation layer traversed by aborehole according to the present invention comprises the steps of

[0009] a) selecting a location in the formation layer;

[0010] b) lowering in the borehole to the location a tool that comprisesa central conduit having an inlet, means for displacing fluids throughthe central conduit, and an optical fluid analyser;

[0011] c) making an exclusive fluid communication between the formationand the inlet of the central conduit;

[0012] d) obtaining a spectrum of the optical density;

[0013] e) calculating a first factor that is the maximum optical densityin a predetermined short-wavelength range multiplied with the length ofthe short-wavelength range, calculating a second factor which is theintegral over the same short-wavelength range of the spectrum,subtracting the second factor from the first factor to obtain ahydrocarbon oil property; and

[0014] f) obtaining the magnitude of the in situ viscosity from the oilproperty using a relation that had been obtained by fitting a curvethrough previously obtained data points comprising the measuredmagnitude of the actual viscosity as a function of the oil property.

[0015] Suitably the difference in step e) is divided by the opticaldensity of the oil peak to obtain a crude oil property.

[0016] The method will now be described by way of example in more detailwith reference to the accompanying drawings, wherein

[0017]FIG. 1 shows the viscosity in centipoise (at in situ pressure andtemperature) on the y-axis as a function of the hydrocarbon oil propertyon the x-axis in arbitrary units; and

[0018]FIG. 2 shows the viscosity in centipoise (at in situ pressure andtemperature) on the y-axis as a function of the crude oil property onthe x-axis in arbitrary units.

[0019] With reference to FIG. 1, we will now discuss the method ofdetermining the viscosity according to the present invention in reverseorder, wherein we start with discussing how the empirical relation isobtained.

[0020] The curve 1 shown in FIG. 1 shows the empirical relation thatfits the data points 2, 3, 4 and 5 obtained from samples taken fromreservoirs in the same geological area. For the sake of clarity, not alldata points have been referred to with a reference numeral.

[0021] A data point was obtained as follows. At first a well was drilledto the formation layer of interest. Then a tool was lowered to the firstof a set of locations in that formation layer. The tool comprises acentral conduit having an inlet, means for displacing fluids through thecentral conduit, and an optical fluid analyser. At the location anexclusive fluid communication was made between the formation and theinlet of the central conduit by extending into the formation a probehaving an outlet that is in direct fluid communication with the inlet ofthe central conduit. Then formation fluid was allowed to enter into thefluid receptacle and the spectrum of the optical density was obtained.

[0022] Then a first factor is calculated, which first factor is themaximum optical density in a predetermined short-wavelength rangemultiplied with the length of the short-wavelength range. Then a secondfactor is calculated, which second factor is the integral over the sameshort-wavelength range of the spectrum. Here the predeterminedshort-wavelength range is the visible light range. Then the secondfactor is subtracted from the first factor to obtain the hydrocarbon oilproperty, HOP.

[0023] Then a sample of the reservoir fluid was taken, and the viscosityof the sample was measured in a laboratory under reservoir conditions.And the measurements gave a data point in FIG. 1.

[0024] To get all data points these data were collected and analysed formore wells in the same geological area.

[0025] Then a curve was fitted through the data, and surprisingly, thedata could be fitted with a considerable accuracy. The equation of thecurve 1 is Visc=(2.164)exp(−0.15(HOP)), with a goodness of fit R² of$0.91,{{{wherein}\quad R^{2}} = \frac{( {\sum\limits_{i = 1}^{n}{( {x_{i} - x} )( {y_{i} - y} )}} )^{2}}{\sum\limits_{i = 1}^{n}{( {x_{i} - x} )^{2}{\sum\limits_{i = 1}^{n}( {y_{i} - y} )^{2}}}}},{{wherein}\quad n\quad {is}\quad {the}}$

[0026] number of data points, (x₁, . . . , x_(n)) is the set of oilproperties, x is the mean oil property, (y₁, . . . , y_(n)) is the setof measurements of the viscosity and y is the mean viscosity. R² is thesquared value of the correlation coefficient.

[0027] We now discuss how the viscosity of an unknown hydrocarbonreservoir fluid that is present in a formation layer of interesttraversed by a borehole is determined in situ.

[0028] At first a tool is lowered to the first of a set of locations inthat formation layer. The tool comprises a central conduit having aninlet, means for displacing fluids through the central conduit, and anoptical fluid analyser. At the location an exclusive fluid communicationis made between the formation and the inlet of the central conduit byextending into the formation a probe having an outlet that is in directfluid communication with the inlet of the central conduit. Thenformation fluid is allowed to enter into the central conduit and aspectrum is obtained.

[0029] Then the optical density spectrum is used to calculate thehydrocarbon oil property, and the oil property is used with theempirical relation to get the viscosity that is required.

[0030] Suitably, the crude oil property is used, this is the hydrocarbonoil property divided by the optical density of the oil peak. The oilpeak is the optical density at a wavelength of about 1 700 nanometer.

[0031] The curve 10 shown in FIG. 2 shows the empirical relation thatfits the data points 2, 3, 4 and 5 obtained from samples taken fromreservoirs in the same geological area. The equation of the curve 10 is

[0032] Visc=(19.8)(COP)^(−1.4), with a goodness of fit R² of 0.96. Thecrude oil property, COP, had been determined by dividing the hydrocarbonoil property, COP, by the optical density of the oil peak.

[0033] Suitably the optical density of the oil peak is corrected bysubtracting from it the base-line optical density.

[0034] In case the hydrocarbon reservoir fluid is a so-called heavy oilthat is relatively viscous, it will be difficult to acquire arepresentative sample of the reservoir fluid. In order to obtain arepresentative sample, the step of making an exclusive fluidcommunication further includes activating a heating device arranged nearthe probe to heat the formation fluid.

[0035] Suitably, the probe is associated with a packer pad in anassembly, and the heating device is placed in the packer pad.Alternatively the heating device is arranged on the tool. The heatingdevice may be a device generating microwaves, light waves or infraredwaves. The heating device may also be an electrical heater, a chemicalheater or a nuclear heater.

[0036] In the above the borehole traversing through the formation wasnot cased, and the exclusive fluid communication was formed by a probeextending into the formation. In case the borehole traversing theformation is cased, the exclusive fluid communication must be made in adifferent way. Thereto, the step of lowering in the borehole to thelocation a tool that comprises a central conduit having an inlet, meansfor displacing fluids through the central conduit, and an optical fluidanalyser now comprises

[0037] 1) making a perforation set through the casing wall into theformation at a location where the communication needs to be established;

[0038] 2) lowering the tool into the borehole to the perforation set,which tool is further provided with an upper and a lower packer arrangedat either side of the inlet of the central conduit, wherein the centralconduit opens below the lower packer or above the upper packer, andwherein the distance between the upper and the lower packer is largerthan the height of a perforation set, and wherein the step of making anexclusive fluid communication comprises setting the packers so that theperforation set is straddled between the packers.

1. Method of determining the viscosity of a hydrocarbon reservoir fluidthat is present in a formation layer traversed by a borehole, whichmethod comprises the steps of a) selecting a location in the formationlayer; b) lowering in the borehole to the location a tool that comprisesa central conduit having an inlet, means for displacing fluids throughthe central conduit, and an optical fluid analyser; c) making anexclusive fluid communication between the formation and the inlet of thecentral conduit; d) obtaining a spectrum of the optical density; e)calculating a first factor that is the maximum optical density in apredetermined short-wavelength range multiplied with the length of theshort-wavelength range, calculating a second factor which is theintegral over the same short-wavelength range of the spectrum,subtracting the second factor from the first factor to obtain ahydrocarbon oil property; and f) obtaining the magnitude of the in situviscosity from the oil property using a relation that had been obtainedby fitting a curve through previously obtained data points comprisingthe measured magnitude of the actual viscosity as a function of the oilproperty.
 2. The method according to claim 1, wherein the difference instep e) is divided by the optical density of the oil peak to obtain acrude oil property.
 3. The method according to claim 2, wherein theoptical density of the oil peak is corrected by subtracting from it thebase-line optical density.
 4. The method according to any one of theclaims 1-3, wherein making an exclusive fluid communication between theformation and the inlet of the central conduit comprises extending intothe formation a probe having an outlet that is in direct fluidcommunication with the inlet of the central conduit of the tool.
 5. Themethod according to claim 4, wherein making an exclusive fluidcommunication further includes activating a heating device arranged nearthe probe to heat the formation fluid.
 6. The method according to anyone of the claims 1-3, wherein the formation is traversed by a casedborehole, wherein step b) comprises b1) making a perforation set throughthe casing wall into the formation at a location where the communicationneeds to be established; b2) lowering the tool into the borehole to theperforation set, which tool is further provided with an upper and alower packer arranged at either side of the inlet of the centralconduit, wherein the central conduit opens below the lower packer orabove the upper packer, and wherein the distance between the upper andthe lower packer is larger than the height of a perforation set, andwherein step c) comprises setting the packers so that the perforationset is straddled between the packers.